Any data processing terminal device, ranging from a simple calculator to a complex point-of-sale terminal, can be viewed as a combination of functional units (controllers, printers, displays, card readers, etc.), each of which has a dedicated function to perform as well as a need to exchange data and control signals with at least certain of the other functional units.
Conventionally, the functional units have been linked through wired connections usually in the form of electrical cables. There are known disadvantages to the use of electrical cables in terminal devices. Such cables can represent a significant portion of the component cost of a terminal device. The time (and labor cost) required for assembly of terminal devices with electrical cables can also be significant. Moreover, electrical cables may be susceptible to electrical noise or stray signals which can produce errors in the data or control information being transmitted between functional units.
To avoid some of these problems, terminals have been proposed which would utilize fiber optic links between at least some of the functional units. Fiber optic links are less susceptible to noise or stray signals but thus far, cost more and are harder to assemble than electrical cables used for the same purposes.
To overcome some of the known problems of electrical or optical cable links between functional units in terminal devices, attempts have been made to use a cable-free chamber to provide optical links between functional units.
U.S. Pat. No. 4,063,083 discloses a system having an optical loop in which a single beam of optical energy is transmitted along a straight line path within a closed chamber from one pluggable card to the next. At each card, the optical energy may be detected, modified and passed on to the next card along the path. Lenses are included for maintaining the beam focus.
While the approach disclosed in this patent avoids some of the problems inherent in the use of electrical or optical cables, certain other problems seem to be created. Since all communications is along a single linear path defined by card-attached optical elements and beam-folding mirrors mounted within the chambers, the terminal device would have to be carefully assembled to maintain the proper beam alignment. Moreover, the arrangement is somewhat inflexible in that the cards themselves must be arranged in series along the beam path.
An alternative approach to closed chamber optical communications is disclosed in co-pending application Ser. No. 362,681, filed Mar. 29, 1982, and assigned to the assignee of this invention. In that application, one or more functional units can be plugged into a frame adjacent a substantially enclosed optical chamber. A transducer on each functional unit can inject optical energy into the chamber through an opening in one wall thereof and can detect optical energy injected by other transducers through similar openings. Within the optical chamber, injected optical signals are diffusely reflected from the interior walls of the chamber, thereby "flooding" the chamber with optical energy. The diffusely reflected signals are received by all functional units which are in place.
While this approach avoids many of the problems of cabled connections and is more flexible than the single beam loop approach discussed above, it is not free of drawbacks. Typically, the coefficient of reflectance of suitable material for the chamber walls is on the order of 0.8, which means that 20% of the optical energy incident on the wall is absorbed at each reflection. A signal which is reflected many times before reaching a desired transducer may be significantly degraded due to this repeated absorption and due to scattering at each reflection.